As an air compressor which has a mechanism of discharging a drain generated after compressing and cooling air, there is one described in Patent Literature 1. This publication describes that “pressure detecting means (first pressure detecting means) 52 for detecting a pressure in a drain discharge circuit 40 at this position is provided between a strainer 42 and a diaphragm 41” in the drain circuit and “when the pressure in the fluid circuit between the diaphragm and the strainer falls to or below a reference pressure set to a predetermined pressure which is less than the pressure when the strainer is not clogged, it is determined that the strainer is clogged.